1. Field of the Invention
The present invention relates to a power source device including a piezoelectric transformer and to an image forming apparatus such as an electrophotographic image forming apparatus using the power source device.
2. Description of Related Art
As a related art power source device for an image forming apparatus employs an electrophotographic method as described in such as Patent Document 1, a piezoelectric transformer has been known for generating the high voltage by an input of the low voltage using a resonance phenomenon of a piezoelectric vibrator and controlling the piezoelectric transformer using an output signal of a voltage-controlled oscillator (hereafter referred to as VCO) to output the high voltage.
Patent Document 1: Japanese Un-examined Patent Application Publication No. 2006-91757
Such a related art power source device, however, has various drawbacks as follows.
First, the related art power source device includes an analog circuitry, such as the VCO or the like, causing an increase in the number of its components.
Second, in a case where the high voltage output around resonant frequency of the piezoelectric transformer is used and where the output voltage is decreased by the load fluctuation, the output voltage cannot be controlled if the frequency of the output voltage is controlled to be lower than the resonant frequency. Consequently, the high voltage output around the resonant frequency cannot be substantially used.
Third, a time constant for controlling the output voltage needs to be selected according to a component constant. Consequently, the prioritization of rise time or leading edge time causes deterioration of the controllability around the resonant frequency. The prioritization of the controllability around the resonant frequency, on the other hand, causes prolongation of the rise time or leading edge time.
Finally, in a case where a control target voltage is relatively row, a circuit structure using an analog oscillator such as the VCO can be influenced by spurious frequency, causing difficulty of controlling the output voltage.
The present invention is proposed in consideration of the aforementioned conventional situations, and provides a power source device and an image forming apparatus including the power source device. According to the power source and the image forming apparatus of the present invention, a sequence generation unit generates an “N” number of sequences, in which each element has substantially the same occurrence rate, with respect to each switching of the pulse by synchronizing with the pulse, thereby capable of obtaining a stable high voltage output having good controllability by obtaining the pulse having low frequency resolving power even if the low frequency clock is divided. Moreover, the constant voltage control can be stably performed from the high voltage output being relatively low to the high voltage output being relatively high around the resonance frequency of the piezoelectric transformer. In addition, the power source and the image forming apparatus of the invention allow a wide output range, so that the output is performed regardless of the environment, thereby obtaining a stable image having substantially no uneven density or a horizontal line. Moreover, the device can be provided with digital circuits, and the number of components can be reduced.